Concept of Watershed:
Watershed is a natural hydrologic entity that encompasses a specific area stretch of land surface where from rainfall or run off flows to a specific defined drain be it a channel/nullah, small stream or river. Watersheds are considered appropriate units for both surveys and assessment of soil and land resources as well as for planning and implementation of various developmental programmes. Owing to optimum interaction and synergetic effect of land and water resources within a watershed, watershed approach, over the period of time, has become popular in various developmental programmes in the country.
The process of delineation of watersheds has a scientific and rational approach. However, for presenting them as units for planning and development, formulation of an appropriate framework for watersheds is a pre-requisite. Such framework shall not only follow a hierarchical system of delineating bigger hydrologic units into watersheds but also include a codification system so that each watershed or units of interventions can be identified as an unique individual entity with clear identifiable linkage with higher hierarchical units, that is, basin, catchments, subcatchment.
The concept of stream order is often followed in geomorphic analysis of natural drainage system. However, framework of watersheds requires a different approach also indicating macro and micro level of delineation so that programmatic interventions units are of manageable sizes. Each of the big drainage systems needs to be divided and sub divided through stages.
The framework applied in this document employs popular terms like basin, catchment, subcatchment, watershed for denoting various stages of delineation from macro to micro levels.
Throughout this document, size of hydrologic entities is governed by the size of the stream or river or the point of interception of the river like a dam, barrage, etc. For example, the size of the Mahanadi river basin is 145 lakh ha whereas the size of the catchment of Hirakud dam on Mahanadi is 83 lakh ha.
Size of sub-watersheds is, however, delineated keeping in view of its relative importance in land and water resource development programmes. On the other hand, size of smallest hydrologic units which are below sub-watersheds are generally governed by viability and working feasibility of implementation of various developmental programmes.